As is well known, electronic semiconductor devices are encapsulated within a package serving as a protection therefor. In particular, reference will be made herein to packages which comprise a plastic material case.
The active portion of such devices is a plate or "die" of a semiconductor material which measures a few square millimeters in surface area and has an electronic circuit, usually an integrated circuit, formed thereon.
So-called power packages are used for those devices which are liable to develop heat in relatively large amounts, as may be due either to high density or large numbers of power components therein, or to operation with large currents, that is for electronic power devices. For this reason, a heat sink is included in the package to dissipate heat generated within the package. The heat sink should have special constructional, e.g. a large heat dissipation area, and thermal features, e.g. high thermal conductivity. In practice, it is an element made of a metal, or good heat conductor, whose mass is definitely larger than that of the die to which it is coupled thermally.
To convey a heat flow generated during the device operation toward the ambient air, the heat sink is only partly embedded within the plastic case, with a major surface of the heat sink being left exposed, i.e. uncovered by the plastic material. An external heat sink may optionally be arranged to contact this exposed surface in order to further enhance the transfer of heat to the package outside.
In certain applications where the device is to operate at a high voltage, e.g. audio apparatus such as car radio sets, high-fidelity devices, and stereo systems for household use, the heat sink is sometimes further connected to the metal framework of the apparatus as well, such as a radio own chassis, to produce a conveniently expanded dissipating surface. However, this arrangement may affect the operation of the integrated circuit in an adverse manner. In fact, the chassis of the apparatus would normally be grounded or at a predetermined electric potential.
Alternatively, the heat sink may be connected to a printed circuit board, or to an external heat sink of larger size, again for the purpose of enhancing the transfer of heat.
In all of the above cases, the voltage between the dissipating outer metal structure, the heat sink proper and the semiconductor material die may damage the power device irreparably.
In general, if discharge between the heat sink and the printed circuit board or the metal pins of the package is to be prevented, the heat sink must be insulated electrically from the metal structures in contact therewith, or from the external heat sink.
Certain prior arrangements for insulating the device electrically, such as the interposition of insulating materials (e.g. mica) between the exposed surface of the internal heat sink and the outside structure providing additional dissipation, result in added complication and cost for their assembly and testing.
Other solutions consist of forming an insulating layer over the exposed surface of the heat sink. Such processes provide, for example, for the deposition of an oxidizing metal layer, followed by an oxidation or by the deposition of an insulating layer by a chemical method. Drawbacks are encountered in carrying out such processes, especially when chemical treatments are used.
The present invention is related in particular to the methods most commonly used for insulation wherein the exposed surface of the heat sink is fully covered with the same plastic material as the package case. The heat sink obtained with these methods is fully embedded within the plastic case during the case molding step. The resultant package is referred to as having an insulated heat sink, it being understood that the insulation provided is electrical in nature.
To best compromise, satisfactorily and functionally, between a good electric insulation and optimum dissipation of the device heat, it is necessary for the layer of plastic material covering said surface of the heat sink to be quite thin.
Referring to FIG. 1, it is shown diagramatically a side view of a typical insulated heat sink power package, with its left-hand portion in section, such as it appears after the molding step. The package is generally and schematically shown at 1.
In this and the following figures, has been considered illustratively the instance of a so-called "single-in-line" package wherein the pins protrude out of only one long side of the package body. This configuration is a frequently adopted one for power packages.
The package 1 for the electronic device comprises suitable supporting and electric interconnection means within a case 2 made of a plastic material, typically a resin. A die of a semiconductor material, on which the device has been formed, is mounted rigidly with such means so as to leave the surface where the circuit is formed unencumbered.
Specifically, for mounting the package to a printed circuit board, a lead leadframe 3 typically comprises, as shown by the cross-sectional portion, a plurality of electric connectors or leads, denoted by 4, which have one end outside the case 2. The leads 4, once bent, will form the package pins.
The structure for supporting the die also functions to dissipate heat generated within the package and includes, for this purpose, a heat sink 5.
The leadframe 3 is connected to the underlying heat sink 5, usually by means of rivets, not shown in the figure. The leadframe 3 is held by the rivets slightly raised above the heat sink 5, and their electric connection is only effected through suitable sunk areas of the leadframe, in this example.
The semiconductor material die, not shown in the figure, is fixed on the top surface of the heat sink 5. For connecting the leadframe 3 electrically to the circuit, the inner ends of the leads are connected by thin metal wires to a corresponding metallized area provided on the exposed surface of the die.
The assembly formed of the leadframe 3 and the heat sink 5 is duplicated for a number of times forming a continuous strip. A peripheral structure of a single leadframe is part of a common holder structure of the whole strip. As shown in the Figure, directly after molding, the peripheral structure, denoted by 6, will remain outside the case 6 while still joining the packages together in one strip.
As can be seen, the plastic case 2 encapsulates the leadframe 3 partially to leave only the ends of the leads 4 on the package outside, and fully encapsulates the heat sink 5.
The bottom surface of the heat sink 5 is covered, in fact, by a resin layer 7. This layer is provided quite thin, so that its thermal resistance can be kept low. In this situation, the transfer of heat from the heat sink to the ambient occurs unhindered. The term "thin" used herein in connection with the thickness of the layer 7 will indicate a negligible thickness compared to that of the plastic material covering the heat sink surface on the side of the die, i.e. at the top in the figure.
Shown in FIG. 2 for the sake of clarity is a top plan view of the package 1 of which FIG. 1 shows a partial cross-section taken along a section poly line A--A. Shown therein are the package for a single device and a portion of the leadframe of an adjacent device.
The leadframe 3 and heat sink 5 are shown in phantom lines in so far as concerning their portions included within the case 2.
The rivets connecting the leadframe 3 to the heat sink 5 are shown at 8. An electric connection between the leadframe 3 and the heat sink 5 is established, in accordance with an exemplary solution described in European Patent Application 545007 by the Applicant, by a conductive leadframe lead 9 which is sunk in the central portion to contact the heat sink 5.
Defined in the leadframe 3 are the metal leads 4, still joined together by transverse interconnecting sections extending from a central region of the leadframe toward the bottom peripheral holder structure 6 with which they are formed integrally.
For a better understanding of the present invention, a brief review of the standard process steps for forming conventional packages may be of assistance in conjunction with the previous figures. Once the leadframe 3 is constrained to the heat sink 5, the corresponding dice are mounted conventionally by placing them directly onto a major surface of the heat sink 5, without the leadframe 3 interposed.
The die is positioned centrally on the heat sink 5, spaced apart from the leads 4. Alternatively, in a modified embodiment, the die is placed onto a central portion of the leadframe 3 which is also connected to the heat sink 5. In the former case, the die is fixed as by soldering, using a low-melting alloy such as a lead/tin alloy, or gluing, using a suitable glue such as an epoxy adhesive.
The ends of the leads 4 which surround the die and are isolated therefrom are then connected electrically using thin metal wire leads, usually gold wire.
The strip, with the dice assembled thereto, is then placed into a mold having corresponding cavities for the individual devices, for injecting an electrically insulating material in a molten state at a high temperature to form the plastic body of the package. This material is typically a synthetic resin, e.g. an epoxy resin. The transfer molding process is carried out in steps through which the temperature is varied gradually to avoid cracking the semiconductor material or in any way impairing the device reliability. After a first cooling step, and subsequent curing steps to promote thorough polymerization of the resin, the series of packages thus formed are removed from the mold.
FIG. 3 illustrates schematically the molding process for the formation of the package shown in the previous figures. In particular, the figure shows a single cavity mold.
A mold for the injection of resin is generally shown at 10. It comprises an upper half or top mold 10a and a lower half or bottom mold 10b, each provided with a corresponding recess. The two halves are disposed with their recesses opposite to each other so as to form a mold cavity into which the resin will be introduced.
The leadframe 3 (labeled in FIGS. 1 and 2) is placed between the two halves 10a and 10b, inside the mold cavity with the ends of the leads 4 (labeled in FIGS. 1 and 2) protruding outside. Also shown in the figure, on the opposite side from the leads 4, is a section of part of the peripheral holder structure 6 of the leadframe 3 which is left outside the mold cavity.
The rivets 8 providing mechanical connection are clearly visible in the figure along with the die 11 located on the top surface of the heat sink 5.
The molten resin is injected through a gate provided in the mold, as shown at 12 in FIG. 3. It can be seen that the gate 12 has its axis substantially horizontal, at the level of the heat sink, and has an opening into the mold cavity which locates in one of the side surfaces, as shown at 13. This location of the gate 12 allows the mold cavity to be filled gradually to its farthest region from the resin entrance, on the right in the figure. In particular, to foster an even inflow of resin, it has been found convenient to have the gate 12 located close to one mold corner, or edge of the package to be formed.
To form the thin layer 7 shown in FIG. 1, it is common practice to create a gap, during the case 2 molding, between the bottom surface of the heat sink and the facing wall of the mold cavity, such as by reducing the heat sink thickness, for example. This gap will be filled with the resin. In other words, the surface 14 of the heat sink and the facing wall 15 of the mold cavity are spaced apart to provide room for the formation of an insulating layer.
Typically, the surface 14 of the heat sink, which is to be covered with the thin insulating layer, is so disposed within the mold as to be the bottom surface. In this way, the resin flow to the region, referenced 16, constituting a slit between the surface 14 and the wall 15, is facilitated.
However, due to the small thickness of the filling region 16 underlying the heat sink 5, it is extremely difficult to fully cover, or cover with plastic material to an even thickness, the bottom surface 14 by such a conventional molding arrangement, even in the preferential embodiment thereof.
To illustrate this drawback, indicated by arrows in FIG. 3 is the main direction of the streamlines for the resin entering the mold cavity. The molten plastic material from the gate 12 is distributed out both toward the upper portion of the cavity, overlying the support and dissipation structure 3 and 5, and toward the lower portion underlying the heat sink 5. However, it will meet a higher flow resistance toward the lower portion of the mold than toward the upper portion thereof. In fact, the downwardly directed passage channel where the resin flow has to be introduced has a significantly smaller cross sectional area.
A flawless homogeneous layer is difficult to obtain, especially where high-dissipation resins are used which contain coarse crystalline fillers. And yet, not even by using a low-viscosity resin can the problem be solved to the point that a uniform thickness can be ensured in a repeatable manner for the resin layer 7 which is to insulate the heat sink.
Even when the resin fully covers the bottom surface 14 of the heat sink 5, the resulting electric insulation is bound to be doubtful and the heat dissipating capability to be degraded. Moreover, the thickness of the plastic material layer under the heat sink 5 cannot be increased since it is necessary that it exhibit low thermal resistance.
To overcome the high resistance to the resin flow and ensure a low rate of defectivity in the resin, the resin is usually injected under a high pressure.
However, just due to the fact that the resin enters the mold cavity under pressure and that, moreover, the amount of resin is greater in the portion above the heat sink, the heat sink, through not being held firmly within the mold, is pushed downwards. This further restricts the resin channel 16, and may on occasions cause the bottom surface 14 of the heat sink to touch the opposite wall 15 of the mold cavity.
The problem is worsened in that the heat sink may undergo deformations during the forming and leadframe mounting steps, so that the heat sink may enter the mold in a slightly concave shape, with its central portion sunk down. In this condition, the central region of the bottom surface 14 of the heat sink is particularly likely to be left uncovered due to downward displacement of the heat sink during the molding process.
This poses the problem of an accurate and well calibrated positioning of the heat sink in the mold during the resin injection and gardening.
In order to ensure a satisfactory level of quality and reliability of the electric insulation, some solutions are known which provide for the use of pairs of clamping pins in engagement with the heat sink and the leadframe, from below as well as from above, to retain the heat sink in the corrected position inside the mold cavity during the molding process.
Such solutions have been applied, for instance, to devices wherein one of the leads is formed integrally with the heat sink, thereby also functioning as a device electrode or terminal. In this case, only the leads are clamped between the two mold halves and, accordingly, keep the structure fixed from one side. On the opposed side, the heat sink is instead completely allowed to move within the cavity during the molding process. In any case, the problems are similar to those discussed in connection with the exemplary structure of FIGS. 1-3.
A first known method employing clamping pins provides for the use of pin pairs fixed inside the mold cavity to block the heat sink and leadframe during the resin injection and until the resin hardens. In this way, the heat sink remains immovable during the molding process. However, when the package is removed from the mold, the volumes occupied by the pins are found uncovered by the resin, and in particular portions of the heat sink bottom surface are left exposed. It becomes necessary to fill these volumes with additional liquid thermosetting resin to thoroughly seal the package and avoid the risk that the device may lack insulation in such regions. Otherwise, the international standards concerning insulation tab and safety would not be met. In addition, the device mounted on the printed circuit board may be destroyed by incidental short circuits with other devices.
Packages made with the above method exhibit reliability and cost problems. In fact, the reliability of the electric insulation of devices thus encapsulated is heavily dependent on the adhesion characteristics at the interface between the liquid sealing resin and the encapsulating resin. The cost of a device sealed with two resins is increased by the addition of a manufacturing step, necessary for dispensing the liquid sealing resin. Furthermore, disadvantages due to the difficulty involved in the control of such dispensation by automated processes, and to the criticalness of the resin dosing should be considered, since any excess of liquid sealing resin which may leak out onto the surface intended to be superimposed to the external dissipation structure would impair the surface planarity and, hence, the proper transfer of heat.
An improved method is described in U.S. Pat. No. 4,888,307 owned by the same assignee as this patent. This known method provides for the insertion, prior to molding, of thermoplastic nails through suitable holes formed in the heat sink, as shown in FIGS. 4a and 4b. In FIG. 4a, the heat sink 5' is shown schematically in a section across the holes, and the nails are designated 17. The upper ends of the nails are riveted, and then the heat sink is placed into the mold 10' as shown in FIG. 4b. During the molding process, the heat sink is clamped by pairs of fixed pins 18.
On removal of the finished package from the mold, both major surfaces of the heat sink remain insulated by plastic material by virtue of the nails 17 provided, thereby avoiding the need for forming additional resin plugs. However, the process complexity is not reduced because additional processing steps are required prior to molding.
A further solution to the problem is described in the aforementioned US patent, and depicted in FIGS. 5a-5c. The proper positioning of the heat sink within the mold cavity is ensured by pairs of retractable locating pins 19. The method comprises the steps of inserting the heat sink 5" and leadframe 3" into the mold cavity, between the mold halves 10"a and 10"b, and blocking them by the pins 19, as shown in FIG. 5a. Thereafter, a first resin injection step is carried out through the gate 12". Once the mold cavity is filled with the resin, the pins 19 are pulled gradually out of the mold while carrying out a second resin injection step to fill the voids left by the pins, as shown in FIG. 5b. The retraction of the pins 19 is effected concurrently with the resin polymerising and hardening. FIG. 5c shows that, after completion of the molding process, the finished package is ejected out of the mold by driving the locating pins 19 and specially provided ejecting pins 20.
In actual practice, during the first step, the resin would have a low viscosity at a predetermined molding temperature, which facilitates its flowing. During this step, the proper positioning of the heat sink 5" is ensured by the pins. During the second step, the cavity fully filled with resin slows the movement of the heat sink 5", which is now supported by the resin having increased viscosity during this step. As can be seen, the formation of the plastic case by this method is completed with the molding process, and requires no additional operations. Furthermore, the use of a single plastic material for the package allows the formation of a case which can better seal the device.
A disadvantage of this method resides, however, in the high cost of the precision mechanism used to drive the pins during the molding process, in the respect of the assembly, testing, and maintenance thereof.
In addition, the reliability of this method rates fairly low because it is impossible to completely and predictably prevent the heat sink from shifting downwards upon retraction of the pins from the mold. Thus, the proportion of packages where, upon complete resin solidification, part of the heat sink bottom surface is left uncovered is quite significant.
It should also be considered that there is a substantial risk of defects developing in the resin at the regions previously occupied by the locating pins. In fact, the encapsulation process provides, as said before, for the resin injection to be carried on at increased viscosity during the second step in order to fill the voids left by the pins. Due to the increased resin viscosity, the resin compacting may turn out unsatisfactory (reduced thickness, porosity, etc.), which results in faulty electric insulation at the locations of the retracting locating pins.
The underlying technical problem of this invention is to provide a method of forming an electronic device package of plastic having an insulated heat sink, whereby optimum position stability can be ensured for the heat sink until the plastic material is fully solidified. The method should also be uniquely simple, requiring no additional steps to molding, to yield a reliable package as concerns electric insulation and thermal dissipation.
Another object is to enable the use, in the molding process, of a simple and versatile apparatus which can be readily implemented and involves very low maintenance costs.